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Simple constant current with op-amp, oscillating at very low currents

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Benta:

--- Quote from: Ian.M on June 01, 2023, 09:49:38 am ---The usual fix for oscillating OPAMP current sinks is to roll-off the HF gain of the OPAMP with local negative feedback.  Try 1K in series with -in and 1nF direct from -in to its output

--- End quote ---
After having had problems with CC LED drivers oscillating in the MHz range, I started to investigate.
The very interesting outcome was, that it's actually the output emitter-follower oscillating, not the op-amp.
There are several articles freely available on the subject, plus a number of IEEE papers (where I only know the titles, members only).
Example:
https://www.hifisystemcomponents.com/downloads/articles/Prevent-Emitter-Follower-Oscillation.pdf

The easiest way to kill it is to include a suitable base resistor. After doing that, all my LED drivers were stable.

These CC circuits are not as harmless as they look.

r6502:
hello DavidAlfa,

have a look at the Keithley K263 current calibrator manual. It can deliver currents from 20mA down to 2pA range. In the manual the schematic is included. It is performing really great.

Manual is available at the TEK website: K263 Manual

Principe of the current source see below:

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